Abstract

Vertical deformation and horizontal strains are analyzed in the central Cascadia margin, or the Juan de Fuca plate segment of the Cascadia subduction, zone to establish the widths of the inter–plate strongly–coupled zone and the transition zone. Local variability of marine terrace elevations, corresponding to marine isotope stages 5a–5e (~80-130 ka), in the south and central Oregon coast (32±23 m–to–10.5±7.4 m mean sea level) and in the northern Oregon and Washington coasts (9.5±3.2 m–to–7.6±2.1 m), constrain the width of the strongly coupled zone (~100 km) from the deformation front or buried trench along the central Cascadia margin in Washington and Oregon. Late Holocene tidal marsh sites (number = 25) in Washington demonstrate three conditions of cyclic coseismic subsidence, representing the first and second zero-isobases (< 0.5 m sea level rise) and the trough of maximum coseismic subsidence (1.0±0.5 m or > 1.5 m abrupt sea level rise) for most rupture events during the last 2.6 ka. Together with paleosubsidence data from the Oregon coast the tidal wetland records in Washington show that the first and second zero–isobases, respectively, reach distances of ~100 and ~200 km from the deformation front in the central Cascadia subduction zone. Global positioning system (GPS) baselines show that across–margin crustal shortening (strain rate -5 × 10^-8 to -1.5 × 10^-6 a^-1) is about one order of magnitude larger than along–margin shortening (strain rate -2 × 10^-8 to -5 × 10^-7 a^-1) near the coast during the last several years. Significant negative strain or shortening in the upper–plate is measured to 200 km distance from the deformation front, thereby establishing an unusually wide coupled zone (~200 km) in the central Cascadia margin. The landward limits of the west coupled zone, central uncoupled zone, and east coupled zone, respectively, could account for long–term uplift of the Coast Ranges, the lack of uplift in the Puget and Willamette forearc valleys, and uplift of the western margin of the Cascade volcanic arc.

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